US President Donald Trump on Tuesday reassured Japan he was committed to a military treaty that both nations have described as a cornerstone of security in Asia, after a media report said he had spoken privately about withdrawing from the pact.

Citing unidentified sources, Bloomberg reported on Monday that Trump had discussed ending the pact which he believed is one-sided because it obligated the US to defend Japan if attacked but did not require Tokyo to respond in kind.

The report said Trump was also unhappy with plans to relocate the US base on Japan's Okinawa island.

"The thing reported in the media you mentioned does not exist," chief government spokesman Yoshihide Suga told reporters in Tokyo when asked about the report.

"We have received confirmation from the US president it is incompatible with the US government policy," he added.

Under the security agreement, the US has committed to defend Japan, which renounced the right to wage war after its defeat in World War II.

Japan in return provides military bases that Washington uses to project power deep into Asia, including the biggest concentration of US Marines outside the US on Okinawa, and the forward deployment of an aircraft carrier strike group at the Yokosuka naval base near Tokyo.

Ending the pact, which also puts Japan under the US nuclear umbrella, could force Washington to withdraw a major portion of its military forces from Asia.

It would also force Japan to seek new alliances in the region and bolster its own defences, which in turn could raise concern about nuclear 

proliferation in the already tense region.

Washington's close ties to Tokyo have also benefited US military contractors such as Lockheed Martin Corp and Raytheon Co, which have sold billions of dollars of equipment to Japan's Self Defense Forces.

On a visit to Japan in May, Trump said he expected Japan's military to reinforce US forces throughout Asia and elsewhere as Tokyo bolsters the ability of its forces to operate further from its shores.